The object of the invention is a transceiver for transmitting and receiving an RF signal on at least two operating frequency ranges.
Mobile communication systems are developing and spreading very fast, and therefore systems of many different standards have been built or are being built in many areas. Therefore there has arisen a need for mobile stations which can be used in more than one system. As examples we can mention the digital systems of the GSM type (Global System for Mobile communications) operating on the frequency ranges 900 MHz, 1800 MHz and 1900 MHz, of which the system with the frequencies 1800 and 1900 MHz are also called DCS and PCN systems. These systems operate on different frequency ranges, but otherwise their specifications are closely related. A problem in realising a transmitter/receiver or transceiver is how to avoid the need for separate transmitter and receiver circuits for all frequency ranges.
From the patent publication EP 653851 there is known a transceiver arrangement which uses one local oscillator having a frequency which is selected between the lower operating frequency range and the higher operating frequency range, so that the same intermediate frequency can be used for the operation on both operating frequency ranges. However, the disadvantage of this solution is that due to the intermediate frequency stages the embodiment is considerably complicate, and the manufacturing costs of the device will be high due to the high number of components.
In a direct conversion receiver, or in a zero intermediate frequency receiver, the radio frequency signal is transformed directly to the baseband without an intermediate frequency. Because no intermediate frequency stages are required the receiver requires only few components, wherefore it is an advantageous solution for many applications, such as mobile stations. Solutions for practical embodiments are described in more detail i.a. in the patent application publication EP 0 594 894 AI.
FIG. 1 shows a previously known block diagram of a mobile station""s transceiver, where the receiver is a so called direct transform receiver. There the RF signal received by the antenna is connected with the switch 104 either to the DCS branch or to the GSM branch of the circuit. When receiving a signal of the DCS frequency range the received signal is supplied in the DCS branch to a bandpass filter 106, an LNA (Low Noise Amplifier) 108 and a bandpass filter 110. Then from this signal the block 112 generates components with a mutual 90 degrees phase shift. The in-phase component I and the quadrature component Q are further supplied via the switches 114 and 134 to the mixers 116 and 136.
The mixing signal to the mixers is obtained from the synthesiser 140 having a frequency which corresponds to the received carrier frequency, whereby the obtained mixing results are the in-phase component and the quadrature component of the complex baseband signal. The baseband signal is further supplied to the AGC (Automatic Gain Control) 137 and the correction block 138 for the voltage difference. Then the signal is further processed in the baseband processing unit, block 139, for the received or the RX signal.
When a GSM signal is received the switch 104 directs the received signal to the GSM branch, which correspondingly has in a series connection a bandpass filter 126, a low noise amplifier 128 and a bandpass filter 130. Then the signal is supplied with the same phase to the mixers 116 and 136. Now the switches 115 and 135 select a signal from the synthesiser as the mixing frequency having a frequency which is divided by two in the block 111. In block 111 there is formed two signals with a mutual 90 degrees phase shift to the mixers 116 and 136. Thus the 90 degrees phase shift required by the mixing is not made on the received signal but on the mixing signal. The complex baseband signal from the mixers is supplied to the processing unit 139 for the received baseband or RX signal.
In a known way the synthesiser 140 comprises a PLL (Phase Locked Loop), which comprises a VCO (Voltage Controlled Oscillator) 141, the output signal of which is amplified by the amplifier 146 in order to generate the output signal. The frequency of the signal provided by the oscillator 141 is divided by an integer Y in the divider 142, and the resulting signal is supplied to the phase comparator 143. Correspondingly, the frequency of the signal generated by the reference oscillator 158 is divided by an integer X in the divider 144 and supplied to the phase comparator 143. The phase comparator outputs a signal which is proportional to the phase difference of said two input signals, whereby the output signal is supplied to an LPF (Low Pass Filter) 145, and the filtered signal will further control the voltage controlled oscillator 141. The above described phase locked loop operates in a known way so that the output frequency of the synthesiser is locked to the frequency coming to the phase comparator from the reference frequency branch. The output frequency is controlled by changing the divisor Y.
In the transmitter section the complex baseband transmission signal or TX signal is processed in the TX signal processing unit 160, from where the complex components of the signal are directed to the mixers 162 and 182, where the carrier frequency signal is generated by mixing the input signal with the mixing signal. If the DCS frequency is used in the transmission then the switches 111 and 161 select the output signal of the synthesiser 140 as the mixing signal. The obtained DCS signal is supplied to the bandpass filter 168, the amplifier 170 and the bandpass filter 172. The generated RF signal is further supplied to the antenna 102 by the switch 180.
If the transmission is on the GSM frequency range, then the mixing signal is generated by dividing the frequency of the output signal from the synthesiser 140 by two in the divider 161, from where there is obtained two mixing signals with a mutual 90 degrees phase shift for the first TX mixer 162 and the second TX mixer 182. The signal at the carrier frequency is supplied by the switches 164 and 184 to the GSM branch, where the in-phase component and the quadrature component obtained from the mixers 162 and 182 are added, block 186. Then there is filtering and amplification in the blocks 188, 190 and 192. The generated RF signal is supplied to the antenna 102 by the switch 180. Thus at the GSM frequency the 90 degrees phase shift is made on the mixing signal and not on the signal at the carrier frequency obtained as a mixing result.
The above mentioned controllable blocks receive their control from a processing unit (not shown in FIG. 1), which can contain for instance a microprocessor and/or a DSP (Digital Signal Processor). Further a mobile station comprises a memory unit associated with the processing unit and user interface means, which comprise a display, a keypad, a microphone and speaker, which neither are presented in FIG. 1.
A problem associated with the solution shown in FIG. 1 is to obtain a sufficiently accurate phase: the accuracy requirements on the phase difference between the I and Q components is of the order of a few degrees. On the other hand the control of the phase accuracy is complicated by the operation on two frequency ranges far from each other. As the phase shift in conventional RC phase shifters depends i.a. on the frequency and the temperature of the component it is difficult to achieve a sufficiently accurate phase over the whole frequency band and in all operating conditions. In addition the phase accuracy of the synthesiser is lower at the higher frequency range, because the output frequency of the VCO is the same as the RX/TX mixing frequency.
Further, known transceiver arrangements observe only the operation on two frequency ranges. However, as the number of systems operating on different frequency ranges is increasing it is desirable to provide transceiver devices operating also on more than two frequency ranges. To this end the prior art has not presented any solution
The object of the invention is to create a simple solution in order to realise a transceiver operating on at least two frequency ranges, so that the above presented disadvantages associated with prior art solutions can be avoided.
One idea of the invention is to use a transceiver based on the direct transform and where the mixing frequency is generated with the aid of the same synthesiser when operating on different frequency ranges. This is preferably realised so that when operating on frequency ranges far from each other the output signal of the synthesiser is divided with different divisors in order to generate the mixing frequency. When operating on frequency ranges which are close to each other the divisor used in the feedback of the synthesiser is advantageously changed. In this way it is possible to realise a transceiver which has several operating frequency ranges close to each other.
With the aid of the invention it is possible to generate, in connection with the division of the synthesiser frequency, two mixing signals with a mutual 90 degrees phase shift, whereby no RC phase shifters are required in the signal line and the obtained phase accuracy is good and independent of the frequency.
As the synthesiser operates on a high frequency in the solution according to the invention it is possible to use a higher frequency in the phase comparator and/or a loop filter operating on a wide band, due to which the synthesiser will have a short settling time. Further, due to the high operating frequency the frequency resolution will be increased, whereby the synthesiser can be controlled to channels with frequencies closer to each other. A transceiver according to the invention can further be realised as a simple transceiver with low manufacturing costs, as the circuits can be easily integrated.
A direct transform transmitter/receiver operating on at least two different frequency ranges, where the first frequency range comprises a first transmission range and a first reception range and where the second frequency range comprises a second transmission range and a second reception range, and whereby
said receiver comprises at least one RX mixer for mixing a received signal to a baseband signal,
said transmitter comprises at least one TX mixer for mixing a baseband signal to a transmission signal at a carrier frequency,
the transmitter/receiver comprises synthesiser means,
to the output of the synthesiser means there is connected first frequency division means for generating a first in-phase RX mixing signal and a first 90 degrees phase shifted RX mixing signal for the RX mixer in order to mix a signal received at the first reception frequency range to a baseband signal, and
to the output of the synthesiser means there is connected second frequency division means for generating a first in-phase TX mixing signal and a first 90 degrees phase shifted TX mixing signal for the TX mixer in order to mix a first baseband TX signal to a first TX signal at the carrier frequency in the first transmission frequency range, is characterised in that the transmitter/receiver further comprises
third frequency division means connected to the output of the synthesiser means for generating a second in-phase RX mixing signal and a second 90 degrees phase shifted RX mixing signal from the output signal of said synthesiser means in order to mix a signal received on a second reception frequency range to a second baseband RX signal, and
fourth frequency division means connected to the output of the synthesiser means for generating a second in-phase TX mixing signal and a second 90 degrees phase shifted TX mixing signal from the output signal of said synthesiser means in order to mix a second baseband TX signal to a second signal at the carrier frequency on the second transmission frequency range.
Preferred embodiments of the invention are presented in the dependent claims.